1. Field of the Invention
The present invention relates to a variable nozzle unit which can alter a passage area for (a flow rate of) gas such as exhaust gas to be supplied to a turbine impeller in turbo rotating machinery such as a variable geometry system turbocharger or a gas turbine, and relates to a variable geometry system turbocharger.
2. Description of the Related Art
In recent years, various variable nozzle units for use in variable geometry system turbochargers have been developed. A general configuration of a conventional variable nozzle unit will be described below.
In a housing of a variable geometry system turbocharger, a shroud ring as a first base ring is provided concentrically with a turbine impeller. A plurality of first supporting holes are formed in the shroud ring at equal intervals in the circumferential direction of the shroud ring. Moreover, a nozzle ring as a second base ring is provided at a position away from and facing the shroud ring in the axial direction of the turbine impeller integrally and concentrically with the shroud ring. A plurality of second supporting holes are formed in the nozzle ring at equal intervals in the circumferential direction of the nozzle ring in such a manner as to match the plurality of first supporting holes of the shroud ring.
A plurality of variable nozzles are disposed between a facing surface of the shroud ring and a facing surface of the nozzle ring at equal intervals in the circumferential direction of the shroud ring (nozzle ring). Each variable nozzle is rotatable in both the forward and reverse directions about an axis parallel to the turbine impeller. Moreover, a first nozzle shaft is integrally formed on a side surface of each variable nozzle on one side in the axial direction. Each first nozzle shaft is rotatably supported by the corresponding supporting hole of the shroud ring. Further, a second nozzle shaft is formed on a side surface of each variable nozzle on the other side in the axial direction integrally and concentrically with the first nozzle shaft. Each second nozzle shaft is rotatably supported by the corresponding second supporting hole of the nozzle ring.
A link mechanism for synchronously rotating the plurality of variable nozzles in the forward and reverse directions is provided on the opposite side of the shroud ring from the facing surface. Synchronously rotating the plurality of variable nozzles in the forward direction (opening direction) increases the passage area of exhaust gas to be supplied to the turbine impeller. Synchronously rotating the plurality of variable nozzles in the reverse direction (closing direction) decreases the passage area of the exhaust gas.
A nozzle supporting structure for supporting the variable nozzle will be described below.
The inner surface of the first supporting hole of the shroud ring has on one side in the axial direction a first bearing portion by which the first nozzle shaft of the variable nozzle is rotatably supported. The inner surface of the second supporting hole of the nozzle ring has a second bearing portion by which the second nozzle shaft of the variable nozzle is rotatably supported. In other words, the variable nozzle is supported on both sides from both sides of the variable nozzle in the axial direction by the first bearing portion and the second bearing portion. The fitting clearance between the first bearing portion and the first nozzle shaft and the fitting clearance between the second bearing portion and the second nozzle shaft are set to the same value to an accuracy of several tens of micrometers.
Meanwhile, in some conventional variable nozzle units, the plurality of second supporting holes are omitted from the nozzle ring, and the second nozzle shafts are omitted from the variable nozzles. In such a case, the inner surface of the first supporting hole of the shroud ring has on both sides in the axial direction two first bearing portions by which the first nozzle shaft of the variable nozzle is rotatably supported. In other words, the variable nozzle is supported on one side from one side of the variable nozzle in the axial direction by the two first bearing portions. The fitting clearance between one of the two first bearing portions and the first nozzle shaft and the fitting clearance between the other of the two first bearing portions and the first nozzle shaft are set to the same value to an accuracy of several tens of micrometers.
It should be noted that conventional techniques relating to the present invention are disclosed in Japanese Patent Application Laid-Open Publications Nos. 2012-102660 and 2010-71142.